https://www.nowcoder.com/questionTerminal/a9d0ecbacef9410ca97463e4a5c83be7

public class Solution {
    public TreeNode Mirror (TreeNode pRoot) {
        if(pRoot == null){
            return pRoot;
        }
        if(pRoot.left == null && pRoot.right == null){
            return pRoot;
        }
        TreeNode temp = pRoot.left;
        pRoot.left = pRoot.right;
        pRoot.right = temp;
        pRoot.left = Mirror(pRoot.left);
        pRoot.right = Mirror(pRoot.right);
        return pRoot;
    }
}